As a device for measuring a flowrate of a compressive fluid such as a gas, a thermal mass flowmeter (also referred to simply as a thermal flowmeter) is known, and it is widely used as a measuring device of an intake air amount in an electronically controlled fuel injection device of an internal combustion engine. In the thermal mass flowmeter, if a foreign substance adheres to a sensor portion, measurement accuracy is extremely lowered and thus, use in a clean gas-flow environment is required in general.
However, in an actual use environment such as an electronically controlled fuel injection device, condensation is generated by fluctuation in an air temperature/humidity, and droplets adhere to the sensor portion in some cases. If the droplets adhere to the sensor portion, a temperature balance is lost, and a part of the sensor portion is locally brought into an overheated state, whereby the sensor portion is thermally deteriorated or the thermal flowmeter itself is broken in some cases.
Thus, in order to prevent a serious failure from occurring even if foreign substances such as droplets adhere to the sensor portion, various technologies have been developed. For example, in Patent Literature 1 (JP 2001-91323 A), in a thermal air flowmeter for internal combustion engine including a resistive heat generation film provided on a thin portion formed on a silicon substrate and a control circuit for controlling an applied voltage to be applied to the resistive film or a conduction current to be conducted, in which the resistive film is disposed in an intake pipe of the internal combustion engine, and heat is emitted to the air through the resistive film, a thermal air flowmeter for internal combustion engine characterized in that, when droplets adhere to the resistive film, the applied voltage is controlled to be smaller than a voltage applied to the resistive film in measurement of a maximum rated measurement range or the conduction current is controlled to be smaller than a current flowing through the resistive film in measurement of a maximum specified measurement range is disclosed. According to PTL 1, by discriminating and detecting a case in which water droplets, oil droplets or the like adhere to the heating resistor portion and a normal case without droplets, a heated state can be limited only when the water droplets, oil droplets or the like adhere to the heat resistor portion without narrowing the original flowrate measurement range and deterioration of the resistor can be prevented, and it is considered that reliability of the thermal air flowmeter can be ensured. Moreover, since heat capacities of the thin portion and the resistive heat generation film are small, it is considered that responsiveness of rising at power-on can be improved.
In Patent Literature 2 (JP 2012-207925 A), a thermal air flowmeter including a sensor portion having a heat generating heater generating heat by conduction and disposed in an air passage, a heater temperature control portion for controlling a temperature of the heat generating heater so that a temperature difference between the air flowing through the air passage and the heat generating heater becomes constant, and a flowrate detection portion for detecting an air flowrate flowing through the air passage on the basis of a radiation amount from the heat generating heater, in which a liquid body adhesion determining means for determining whether or not a liquid body such as moisture, oil or the like adheres to the sensor portion is provided, and this liquid body adhesion determining means receives an input of a signal relating to an air flowrate from the heater temperature control portion and the flowrate detection portion and when the respective signals both indicate abnormal values departing from predetermined ranges, it determines that the liquid body adheres to the sensor portion is disclosed. According to PTL 2, since a first signal and a second signal relating to the air flowrate, respectively, are obtained from the heater temperature control portion and the flowrate detection portion and adhesion of the liquid body to the sensor portion is determined on the basis of the two signals, accuracy of the determination is high, and it is considered that adhesion of the liquid body to the sensor can be determined with high possibility.